Method and measurement apparatus for determining the transition impedance between two parts of a subdivided neutral electrode

ABSTRACT

A method and a measurement apparatus are provided for determining the transition impedance between two electrode parts of a subdivided neutral electrode used in high-frequency surgery. These makes it possible for the purely capacitive component of the transition impedance to be measured. For this purpose a resonant-frequency shift is measured, which occurs when a basic resonant circuit is expanded to an expanded resonant circuit by incorporating the two electrode parts into it in parallel. In particular, in order to determine the basic resonant frequency of the basic resonant circuit and/or the sample resonant frequency of the expanded resonant circuit, the phase shift between current and voltage is measured and the frequency is adjusted until current and voltage are in phase.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a section 371 of International Application No.PCT/EP2005/005512 filed on May 20, 2005, which was published in theGerman language on Dec. 8, 2005, under International Publication No. WO2005/115262 A1 and the disclosure of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

The present invention relates to a method of determining the transitionimpedance between two parts of a subdivided neutral electrode used inhigh-frequency surgery. In addition, a measurement apparatus fordetermining the transition impedance between two parts of a subdividedneutral electrode used in high-frequency surgery is described, whichcomprises a resonant circuit into which the two electrode parts can beincorporated in parallel as well as a current-supply device by means ofwhich an alternating measurement voltage can be impressed into theresonant circuit.

In high-frequency surgery high-frequency electrical energy is suppliedto the tissue to be treated. A distinction is generally made between amonopolar and a bipolar mode of employing the high-frequency current.

For monopolar use only one active electrode is provided, to which ahigh-frequency alternating voltage is supplied. Furthermore, it isnecessary to apply a neutral electrode to a large area of the patient'sbody, by means of which the circuit is closed owing to current flowthrough tissue between the active and neutral electrodes. The shape ofthe active electrode depends on the particular purpose for which it isemployed. The surface area of the active electrode by way of whichalternating current is conducted into the tissue is relatively small, sothat in the immediate surroundings of the active electrode there is ahigh current density and hence also a large amount of heat is developed.

The current density decreases rapidly at progressively greater distancesfrom the active electrode, insofar as considerable differences in tissueconductivity do not increase the current density at other places in thebody. The alternating currents supplied to the active electrode areconducted away through the neutral electrode. Accordingly, care shouldbe taken to place the neutral electrode in contact with the patient'sbody over a large area, so that it presents only a slight transitionresistance to the high-frequency alternating current.

For bipolar use two active electrodes are provided, between which thetissue to be treated is enclosed. The electrical circuit is closed bythe tissue lying between the two active electrodes, which thus becomesheated when a high-frequency alternating voltage is applied. During thisprocess the major proportion of the current flows between the two activeelectrodes, but in case of aberration there may be some current flow inneighboring parts of the patient's body. In order to conduct suchdiverted currents away from the body over a large area, so that they donot cause any undesired burns, even when bipolar instruments are used insome cases a neutral electrode is employed, which again should be placedin contact with the patient's body over a large area. The neutralelectrode prevents an elevated current density in parts of the bodyother than that between the active electrodes, and avoids undesiredburns.

If part of the neutral electrode should become separated from thetissue, so that the current flow is restricted to those parts of theneutral electrode that remain in contact, higher current densities andhence burning of the tissue can result. As is shown by the followingreferences to the state of the art, various monitoring systems are knownby means of which the transition resistance of the neutral electrode canbe evaluated.

A high-frequency surgical appliance with a neutral electrode dividedinto two parts and circuitry for measuring the resistance between thetwo parts of the neutral electrode is described in the document DE-AS 1139 927. In this appliance there is provided an accessory direct-currentcircuit with a source of direct current, to which the two electrodeparts are connected in series, so that the only connection between thetwo electrode parts consists of the patient's body. If the ohmicresistance measured between the two electrode parts exceeds a certainlimiting value, the high-frequency generator of the surgical applianceis switched off and/or an alarm system is triggered.

A control circuit for controlling the output of a high-frequencygenerator in dependence on the measured high-frequency current flowingbetween an active electrode and a neutral electrode is known from U.S.Pat. No. 3,913,583. The high-frequency current intensity depends on theapparent resistance between active electrode and neutral electrode, sothat the output of the high-frequency generator is ultimately regulatedin dependence on the apparent resistance.

Furthermore, from U.S. Pat. No. 3,933,157, U.S. Pat. No. 5,087,257 andWO 9619152 neutral-electrode monitoring systems are known by means ofwhich the apposition of a two-part neutral electrode to a patient isevaluated by measuring the apparent resistance between the two parts ofthe neutral electrode. For this purpose a measurement circuit isprovided into which the two electrode parts are incorporated in such away that the measurement circuit is closed by way of the part of thepatient's tissue that is situated between the two electrode parts. Todetermine the apparent resistance, an alternating voltage is applied tothe measurement circuit.

From U.S. Pat. No. 4,200,104 a monitoring system for a two-part neutralelectrode is known that is intended to detect a separation of part ofthe neutral electrode from the patient by measuring the capacitancebetween the two parts of the neutral electrode. The proposed circuitryfor capacitance measurement comprises a monostable multivibrator to theinput of which is delivered a signal at constant frequency. The twoelectrode parts of the neutral electrode are incorporated into themultivibrator circuitry in such a way that the capacitance between thesetwo electrode parts influences the pulse width of the output signal fromthe monostable multivibrator. The capacitance is to be found byevaluating this pulse width, and this information is then used to drawconclusions about the area over which the neutral electrode is incontact with the patient. However, the pulse width of the multivibratorcircuit is also influenced by the ohmic resistance between the twoelectrode parts, which can also change depending on the contact area ofthe neutral electrode. Separation of the influences exerted by thecapacitance from those exerted by the ohmic resistance seems to beimpossible with the circuitry proposed here.

DE 197 14 972 A1 also describes an apparatus for monitoring theapplication of a bipartite neutral electrode. This apparatus comprisesan impedance sensor that detects the transition resistances at thesurface of each of the two electrode parts connected in series. Theapparatus is galvanically separated from the electrode-part surfaces bymeans of a transformer. Measurement of the transition resistance at thepatient makes use of the fact that this resistance is in each casedisposed parallel to the transformer and to a capacitance, andaccordingly a parallel oscillating circuit is formed. By triggering thedamped oscillating circuit at its resonant frequency, the voltage isdetermined solely by the transition resistance at the patient.Accordingly, the voltage measured at the resonant frequency allowsconclusions to be drawn about the transition resistance at the patient.

The transition between the body of the patient and the neutral electrodeopposes the high-frequency alternating current not only by an ohmicresistance, but also by a capacitive resistance determined by chargingeffects. The above-mentioned monitoring systems in the state of the artare limited to detecting either the ohmic resistance or the apparentresistance in this transition impedance.

The document U.S. Pat. No. 4,200,104 also describes a device formeasuring capacitance and thereby estimating the contact area of theneutral electrode, but the capacitance measurement by the measurementdevice proposed there is influenced by the ohmic component of thetransition impedance.

BRIEF SUMMARY OF THE INVENTION

The object of the present invention is to provide a measurementapparatus and a method that make possible an improved evaluation of theextent to which the neutral electrode is in contact with the patient.

According to one aspect of the invention there is provided a method ofdetermining the transition impedance between two electrode parts of asubdivided neutral electrode for use in high-frequency surgery, byproviding a resonant circuit into which said two electrode parts areincorporated in parallel with one another such that the is a transitionimpedance between them; measuring a resonant-frequency shift in saidresonant circuit; and determining the purely capacitive component of thetransition impedance using said resonant-frequency shift. According toanother aspect of the invention there is provided a measurementapparatus for determining the transition impedance between two electrodeparts of a subdivided neutral electrode for employment in high-frequencysurgery, comprising a resonant circuit within which said two electrodeparts are connected in parallel; a current-supply device adapted toimpress a measurement alternating voltage into the resonant circuit; aphase-measuring device; and a control means adapted to determine thephase of the current and of the voltage by means of the phase-measuringdevice in order that the frequency of the current-supply device can becontrolled by the control means in such a way that current and voltageare in phase.

In the present invention the fact that the transition impedance is madeup of both an ohmic and a capacitive resistance is taken into account.The transition impedance is represented in vector form as the vector sumof a real vector for the ohmic resistance and a complex vector for thecapacitive resistance; the value of the transition impedance is heretermed “apparent resistance”.

Because the capacitive resistance is frequency-dependent, measurement ofthe apparent resistance at a test frequency does not in itself providereliable information about the magnitude of the apparent resistancewhile high frequencies are being employed. For that purpose, it would benecessary to know the relative contribution of the capacitive resistanceto the apparent resistance. Accordingly, the invention is based on theprovision of a method and a measurement apparatus by means of which theohmic contribution and the capacitive contribution to the transitionimpedance can be measured separately. In particular, detection of thepurely capacitive part of the transition impedance makes it possible toinfer the size of the contact area between the neutral electrode and thepatient's body. Furthermore, from the capacitive resistance measured ata test frequency it is possible to calculate the capacitive resistanceduring high-frequency employment, which provides the surgeon withadditional valuable information enabling the current amplitude andfrequency to be adjusted to suit the intended high-frequency usage.

To determine the transition impedance it is advantageous to use asubdivided neutral electrode, in particular a neutral electrode dividedinto two parts in such a way that the two electrode parts areelectrically insulated from one another and both are placed in contactwith the tissue, in particular the skin, of the patient. The twoelectrode parts can be incorporated into a measurement circuit so thatthey are electrically connected only by way of the patient's tissue. Thetransition impedance between the two electrodes is determined by the twopartial impedances at the places where the two electrode parts makecontact with the patient's tissue as well as by the conductivity of thetissue situated between the two electrode parts. Accordingly, thetransition impedance between the two electrode parts can be regarded ingeneral as a measure of the degree to which the neutral electrode makescontact with the patient's tissue.

A method in accordance with the invention is therefore designed fordetermining the transition impedance between two electrode parts of asubdivided neutral electrode for use in high-frequency surgery, and ischaracterized in that with this method the purely capacitive componentof the transition impedance is determined.

For determining the capacitive component of the transition impedance aresonant circuit or parallel resonant circuit is employed, in which aninductance is provided in parallel to a capacitance. This makes use ofthe fact that the resonance condition of the resonant circuit depends onits capacitance and inductance, as well as its ohmic resistance, ifpresent. The resonant frequency changes when the two electrode parts areconnected in parallel to the resonant circuit, so that the transitionimpedance is likewise parallel to the resonant circuit. With the methodin accordance with the invention the capacitive portion of thetransition impedance is found by measuring the alteration of resonantfrequency caused by connecting the two electrodes in parallel to theresonant circuit.

When the purely capacitive component of the transition impedance is tobe determined, it is preferable to proceed according to the followingsteps. For the first measurement a resonant circuit should be madeavailable that is not connected to a transition impedance; this will betermed the basic resonant circuit. It comprises a measurement inductanceand a coupling capacitance. The coupling capacitance is preferablyprovided by the coupling capacitors in general use, one of which is tobe disposed in each branch of a branched lead connected to the neutralelectrodes, and which together should help to make sure that the currentis uniformly distributed between the two electrode parts. Themeasurement inductance is parallel to the coupling capacitance andserves to complete the resonant circuit. The component used to providemeasurement inductance is preferably a coil, the inductance of which isappropriate for adjusting the basic resonant circuit to a suitableresonant frequency. Whereas the coupling capacitors are preferablydisposed in the basic resonant circuit and in the high-frequencycircuit, the measurement inductance is preferably disposed only in thebasic resonant circuit and not in the high-frequency circuit.

The first measurement thus involves a resonant frequency of theabove-mentioned basic resonant circuit, which hereinafter will be termed“basic resonant frequency”.

For the second measurement the basic resonant circuit is expanded byadding, in parallel, the transition impedance. In the second measurementthe resonant frequency of this expanded resonant circuit is measured;this will hereinafter be termed the “sample resonant frequency”.

The procedure for finding the capacitive component of the transitionimpedance is based on a model according to which the transitionimpedance can be sufficiently well represented by a purely ohmicresistance positioned within the circuit in parallel to a purelycapacitive resistance. This model provides a good approximation to theactual transition impedance. On the basis of this model the capacitivecontribution to the transition impedance can be found from the basicresonant frequency, the sample resonant frequency and the measurementinductance.

Preferably for calculation of the capacitive resistance the associatedcapacitance C_(x) of the transition between the two electrode parts iscalculated from the measured basic resonant frequency f₁, the measuredsample resonant frequency f₂ and the measurement inductance L. For theresonance condition of the basic resonant circuit the following formulaapplies:

${{2\pi \; f_{1}C_{0}} - \frac{1}{2\pi \; f_{1}L}} = 0$

where C₀ is the coupling capacitance of the basic resonant circuit.

For the resonance condition of the expanded resonant circuit thefollowing formula applies:

${{2\pi \; {f_{2}\left( {C_{0} + C_{X}} \right)}} - \frac{1}{2\pi \; f_{2}L}} = 0$

The capacitance C_(x) of the transition between the two electrode partsis found by combining these two resonance conditions with one another:

$C_{X} = {\frac{1}{4\pi^{2}L}\left( \frac{f_{1}^{2} - f_{2}^{2}}{f_{1}^{2}f_{2}^{2}} \right)}$

The capacitive resistance X_(c) for a particular frequency f iscalculated by the following formula:

$X_{C} = \frac{1}{2\pi \; {fC}_{X}}$

In the resonance case of the basic resonant circuit or the expandedresonant circuit, current and voltage are in phase within an electricallead supplying the relevant resonant circuit. This phase requirement forthe resonance case is preferably imposed for determining the basicresonant frequency and the sample resonant frequency. In particular itis proposed that for measuring the basic resonant frequency of the basicresonant circuit or the sample resonant frequency of the expandedresonant circuit, a measurement alternating voltage is applied to thebasic resonant circuit or the expanded resonant circuit, and the phaseshift between current and voltage is measured in a lead supplying thecorresponding current. The frequency of the measured alternating voltageas adjusted until current and voltage are in phase, and accordingly theassociated resonant circuit is in resonance. In this process thefrequency can be varied by scanning a specified frequency range, untilthe resonance condition has been achieved. Preferably, however, acontrol means is provided that adjusts the frequency of the measurementalternating voltage to the resonant frequency in dependence on themeasured phase relations according to a control algorithm, in particulara proportional-integral control algorithm.

Additional advantages are obtained when the measurement alternatingvoltage is generated by a constant-current source that delivers asquare-wave alternating-current signal with constant current amplitudeto the basic resonant circuit and/or to the expanded resonant circuit.It is particularly advantageous for the frequency of thealternating-current signal to be adjustable by means of a timer signalthat in particular is controlled by a control means, so that the zerocrossing and the frequency of the alternating-current signal are alreadyknown by way of the timer signal.

Another known feature is that when the basic or the expanded resonantcircuit is in the resonance situation, the impedance of the circuit isdetermined exclusively by the ohmic resistance. In this resonance case,therefore, the voltage across the associated resonant circuit ismaximal. This resonance condition can be exploited to measure the basicresonant frequency of the basic resonant circuit or to measure thesample resonant frequency of the expanded resonant circuit. For thispurpose a measurement alternating voltage should be applied to therelevant resonant circuit and the voltage across the said circuit isthen measured. The frequency of the measurement alternating voltage isadjusted until the voltage reaches a maximal value. Preferably aconstant-current source is used as current source.

As has already been discussed above, it is desirable to make available amethod and a measurement apparatus that can be used to measureseparately the ohmic component and the capacitive component of thetransition impedance. Accordingly, the method will preferably alsoinclude determination of the ohmic component of the transitionimpedance. As a result, the transition impedance is also known, as wellas the relative contributions of the ohmic resistance and the capacitiveresistance to the transition impedance, so that it is possible to make awell-founded assessment of the extent of contact between patient andneutral electrode. In particular, the phase angle of the transitionimpedance can be calculated on the basis of the ohmic and the capacitivecomponents.

The ohmic resistance is determined by utilizing the fact that in thecase of resonance of the basic resonant circuit or the expanded resonantcircuit, the transition impedance is given exclusively by the ohmicresistance. If in the resonance case the current in a lead connected tothe expanded resonant circuit and the voltage across the expandedresonant circuit are measured, from these values the ohmic resistance ofthe transition impedance can be calculated directly, insofar as theohmic resistance of the basic resonant circuit is negligibly small. Theohmic resistance is found by dividing a peak value of the voltage signalby a peak value of the current signal. On the other hand, if the basicresonant circuit exhibits a considerable ohmic resistance, this can bedetermined analogously in the basic resonant circuit.

Because the electrode parts can become detached even duringhigh-frequency employment, it is recommended that the transitionimpedance be monitored by measuring the sample resonant frequency of theexpanded resonant circuit at certain time intervals. As a rule, thisrequires a brief interruption of the high-frequency operation.

It is furthermore advantageous for it to be possible to detect which ofthe two electrode parts is making the worse contact. For this purpose,especially during employment for high-frequency surgery, it ispreferable to measure the current flowing away from each of theelectrode parts independently.

Regarding the apparatus, the invention relates to a measurementapparatus for determining the transition impedance between the two partsof a subdivided neutral electrode for use in high-frequency surgery. Themeasurement apparatus comprises a resonant circuit in which the twoelectrode parts can be arranged in parallel, and a current-supply deviceby means of which a measurement alternating voltage can be impressed onthe resonant circuit.

Thus when the idea underlying the invention is applied to a measurementapparatus, said measurement apparatus is designed to determine thepurely capacitive component of the transition impedance. For thispurpose there are provided a control means and a phase-measurementdevice, and the current-supply device, control means andphase-measurement device are designed so that the phase-measurementdevice signals to the control means the phase of the current and voltageof the measurement alternating current applied to the resonant circuit,and the frequency of the current-supply device can be controlled by thecontrol means in such a way that current and voltage are in phase.

Costs are reduced when the measurement apparatus is operated at the“patient's circuit potential”. Then there is no need for a galvanicseparation between parts of the measurement apparatus and the “patient'scircuit potential”, which is ordinarily implemented by expensivetransformers. Only a galvanic separation of the current-supply deviceand other parts of the measurement apparatus from the intermediatecircuitry of the high-frequency surgical appliance is required, and thiscan be implemented for instance by DC/DC converters and optoelectroniccouplers. Hence in accordance with the invention the measurementalternating voltage is fed directly from the current-supply device intothe resonant circuit.

When the electrode parts are not included in the circuitry, the resonantcircuit corresponds to the basic resonant circuit and comprises acoupling capacitance and a measurement inductance parallel thereto. Asmentioned above with reference to the method in accordance with theinvention, the coupling capacitors, one of which is disposed in eachbranch of the bifurcated lead connected to the neutral electrode, areused to provide coupling capacitance, whereas the measurement inductanceis preferably not disposed within the high-frequency circuit.

Furthermore, it is advantageous for the resonance condition of theresonant circuit to be determinable with reference to the phase relationbetween current and voltage, in particular within a lead connected tothe resonant circuit. For this purpose the current-supply device ispreferably a constant-current source, the frequency of which can be setby the control means, by way of a timer signal, and which generates asquare-wave alternating current as output.

Accordingly, the zero crossing of the current and the applied frequencycan be detected by the control means with reference to the timer signal.Preferably an analog comparator is provided, by way of which the zerocrossing in a lead connected to the resonant circuit can be determinedby the control means, so that the control means can detect the phasedifference between current and voltage by way of the timer signal andthe analog comparator.

In particular for determining the ohmic resistance there is preferablyprovided a voltage-measurement device for measuring the voltage acrossthe resonant circuit, so that a peak value of the voltage can bemeasured under the resonance condition of the resonant circuit whentransition impedance is included in the circuit.

In particular for evaluating which of the two electrode parts is makingbetter contact, at every branch of the lead running to an electrode partof the neutral electrode there is disposed a current sensor by means ofwhich the current flowing away from the associated electrode part can bemeasured.

Additional advantageous embodiments of the proposed measurementapparatus will to a great extent be evident from the above explanationsof the method in accordance with the invention in terms of the essentialfeatures of the invention as well as their advantageous embodiments,converted to an implementation in the measurement apparatus according tocircuit technology.

The invention further relates to a high-frequency surgical appliancewith at least one active electrode and one subdivided neutral electrodethat comprises a measurement apparatus in accordance with the inventionfor determining the transition impedance between two electrode parts ofthe subdivided neutral electrode.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe invention, will be better understood when read in conjunction withthe appended drawing. For the purpose of illustrating the invention,there is shown in the drawing embodiments which are presently preferred.It should be understood, however, that the invention is not limited tothe precise arrangements and instrumentalities shown.

In the drawing:

FIG. 1 shows the circuit arrangement of a measurement apparatus inaccordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

With the measurement apparatus 2 shown here, both the capacitivecomponent and the ohmic component of the transition impedance betweentwo electrode parts 4, 6 of a neutral electrode 8 can be measured. Thetwo electrode parts 4, 6 are electrically insulated from one another,such that when both electrode parts 4 and 6 are placed in contact withthe body of a patient, an electrical connection between the twoelectrode parts 4, 6 is produced by the patient's tissue. As a rule thetransition impedance between the two electrode parts 4, 6 is measuredwhile the electrode parts 4, 6 are in contact with the patient. However,it is also possible for a direct transition impedance to be measured byplacing the electrode parts 4, 6 in contact with one another, in such away that the contact surfaces of the two electrode parts 4, 6 are incontact with one another and the electrical connection between them ismediated, for instance, by an intervening layer of gel.

In addition the neutral electrode is provided with an electrical lead10, which at a branch point 12 is split into two branches 14, 16. Eachof the branches 14, 16 runs to one of the electrode parts 4, 6, so thatan electrical charge at the electrode parts 4, 6 can be conducted awaythrough the branches 14, 16 and the neutral-electrode lead 10 to thegrounded part of a high-frequency surgical appliance, in particular ahigh-frequency current supply means. In each branch 14, 16 of theneutral-electrode lead 10 a coupling capacitor 18, 20 is disposed, sothat the coupling capacitors 18, 20 serve to distribute the currentuniformly to the two electrode parts 4, 6.

The electrode parts 4, 6 of the branches 14, 16, together with thecoupling capacitors 18, and the neutral-electrode lead 10 thusconstitute part of a high-frequency electrical circuit. Duringhigh-frequency employment of the equipment, high-frequency alternatingcurrents flow through these components.

The coupling capacitors are combined in parallel with a measurement coil22 to form a basic resonant circuit 24 of the measurement apparatus 2.The two parallel electrode parts 4, 6 can be incorporated in parallelinto the basic resonant circuit 24, so as to expand the basic resonantcircuit 24 to form an expanded resonant circuit 25 by introduction of atransition impedance between the two electrode parts 4, 6.

From a current-supply device 28, by way of a first lead 26, ameasurement alternating voltage can be impressed directly into the basicresonant circuit 24 or into the expanded resonant circuit 25. Acapacitor 30 in the first lead 26 serves as a barrier to direct current.A second lead 32 runs from the basic resonant circuit 24 or the expandedresonant circuit 25 to ground, so that the basic resonant circuit 24 orthe expanded resonant circuit 25 is positioned in series between thefirst lead 26 and the second lead 32.

The measurement apparatus 2 further comprises a control means in theform of a microcontroller 34, which is designed for controlling thefrequency of the current-supply device 28, for detecting the zerocrossing of current and voltage in the first lead 26, for monitoring thevalue of the voltage in the lead 26, and for monitoring the partialcurrents flowing away from the two electrode parts 4, 6.

The current-supply device 28 is a voltage-controlled constant-currentsource that emits a rectangular current signal, the frequency of whichcan be adjusted by means of a timer signal. The current-supply device 28is preferably designed to send out voltages amounting to at most ±10V.For controlling the frequency the microcontroller 34 is provided with atimer 36 that sends a timer signal to the current-supply device 28 inorder to control the frequency of the measurement alternating voltage.The measurement apparatus 2 further comprises a level adjustor 38controlled by the microcontroller 34, which enables various measurementcurrents and hence various measurement regions to be adjusted. As aresult, even the high transition impedance of a neutral electrode 8 thathas not been placed in contact with a patient can be measured; in thiscase the electrical connection between the two electrode parts 4, 6 canbe produced, for example, by disposing a gel bridge between the twocontact surfaces of the electrode parts 4, 6.

The microcontroller 34, by way of an analog comparator 40, detects thezero crossing of the voltage in a lead 26 supplying the basic resonantcircuit 24 or the expanded resonant circuit 25. Accordingly, themicrocontroller 34 has access to information regarding the phaserelation between current and voltage. The microcontroller 34 is designedso that according to a proportional-integral control algorithm it altersthe output frequency of the current-supply device 28, by way of thetimer signal, in such a way that the phase difference between currentand voltage becomes zero. When current and voltage are in phase with oneanother, the basic resonant circuit 24 or the expanded resonant circuit25 is being operated at its particular resonant frequency, i.e. in itsbasic resonant frequency or in its sample resonant frequency,respectively.

The measurement apparatus 2 is operated at the patient circuitpotential. However, with respect to an intermediate circuit of thehigh-frequency surgery appliance the current-supply device 28 and themicrocontroller 34 are galvanically separated. This is achieved by aDC/DC converter 42 and optoelectronic coupler 44.

With the circuitry associated with the measurement apparatus 2 describedabove, the capacitive component of the transition impedance between thetwo electrode parts 4, 6 can be measured in accordance with theinvention.

While the electrode parts 4, 6 are not in contact with the patient, themagnitude of the impedance between the electrode parts 4, 6 is such thatthe basic resonant circuit 24 is not influenced by the electrode parts4, 6. Accordingly, when the electrode parts 4, 6 are not in contact withthe patient, the basic resonant frequency of the basic resonant circuit24, which is derived from the coupling capacitance C₀ of the couplingcapacitors 18, 20 and the measurement inductance L of the measurementcoil 22, can be determined.

For this purpose the microcontroller 34 detects the phase differencebetween current and voltage in the measurement alternating voltageapplied to the basic resonant circuit 24 and according to theproportional-integral control algorithm alters the output frequency ofthe current-supply device 28 in such a way that the phase differencebetween current and voltage is zero. By referring to the time signal,therefore, the basic resonant frequency f₁ of the basic resonant circuit24 is known.

When the electrode parts 4, 6 are in contact with the patient, theimpedance between them is the transition impedance, which is determinedby the measurement device 2.

For this purpose a measurement alternating voltage is applied to theexpanded resonant circuit 25, which is formed by the transitionimpedance between the electrode parts 4, 6, the coupling capacitance C₀of the coupling capacitors 18, 20 and the measurement inductance L ofthe measurement coil 22. The microcontroller 34 detects the phasedifference between current and voltage and, again according to theproportional-integral control algorithm, alters the output frequency ofthe current-supply device 28 in such a way that the phase differencebetween current and voltage becomes zero. By reference to the timersignal, the sample resonant frequency of the expanded resonant circuit25 is known.

The capacitance C_(x) of the transition between the two electrode parts4, 6 is calculated according to the following equation:

$C_{X} = {\frac{1}{4\pi^{2}L}\left( \frac{f_{1}^{2} - f_{2}^{2}}{f_{1}^{2}f_{2}^{2}} \right)}$

where f₁ is the basic resonant frequency and f₂ is the sample resonantfrequency.

The capacitive resistance of the transition for a particular frequency fis found by the following equation:

$X_{C} = \frac{1}{2\pi \; {fC}_{X}}$

The above calculation is based on the assumption that the transitionimpedance can be described by a parallel circuit consisting of an ohmicresistance R_(x) and a capacitance C_(x).

For determining the ohmic component of the transition impedance themeasurement apparatus 2 comprises a circuitry part 46 to measure thevoltage across the basic resonant circuit 24 and/or the expandedresonant circuit 25. The circuitry part 46 comprises a peak-voltagelimiter 48 to suppress high-frequency interference, as well as apeak-value detector consisting of a synchronous rectifier 50 and alow-pass filter 52. The synchronous rectifier 50 is driven by amicrocontroller 34 by way of a signal derived from the timer signal. Themicrocontroller 34 further comprises an analog/digital converter 54 anda UART (universal asynchronous receiver-transmitter) 56, so that themeasured signal from the circuitry part 46 is digitized by the A/Dconverter 54 and by means of the UART 56 is transmitted to a controldevice (not shown) of the high-frequency surgical appliance.

The ohmic resistance R_(x) of the transition impedance between the twoelectrodes is obtained from the following equation:

$R_{X} = \frac{U_{S}}{I_{S}}$

where U_(s) is the peak value of the alternating voltage across theresonant circuit and I_(s) is the peak value of the current intensity.The peak value of the alternating voltage can be measured by way of thecircuitry part 46, whereas the peak value of the current intensity I_(s)is known from the setting of the current-supply device 28, which isconstructed as a constant-current source.

The measurement apparatus 2 shown here is additionally designed tomeasure the high-frequency current flowing away from each of theelectrode parts. These partial currents are detected, in particularduring a high-frequency application, by the current sensors 58, 60disposed in the branches 14, 16 of the neutral-electrode lead 10. Theoutput signal from each of the current sensors 58, 60 is sent to anassociated peak-value detection circuit 62, 64, whereupon an outputsignal from the peak-value detection circuits 62, 64 is sent to the A/Dconverter 54. Accordingly, the partial currents flowing away over theindividual electrode parts 4, 6 can be detected by the microcontroller34. Furthermore, once the measured values regarding the partial currentshave been digitized by the A/D converter 54, the UART 56 makes themavailable for evaluation, in series, to the control device (not shown)of the high-frequency surgical appliance.

Because the invention provides for monitoring of the symmetry of thehigh-frequency partial currents that appear in the two branches 14, 15of the neutral electrode lead 10 during a high-frequency application,loss of contact of one electrode part 4, 6 can be detected. The symmetrycan be evaluated by establishing the relationship of the two partialcurrents. By summation of the partial currents the total HF currentflowing through the neutral electrode can be calculated. In the case amonopolar high-frequency application, this total current can assistevaluation of the plausibility of a measured value for the outputcurrent of the active electrode, and comparison with the latter canenable conclusions to be drawn about the level of a leakage current.

The invention is not limited to the embodiment shown as an example inFIG. 1. Instead the invention results from an expert overallconsideration of the claims, the description, the exemplary embodimentand the variants mentioned below, which are intended to provide a personskilled in the art with indications of additional alternativeembodiments.

In particular the sequence in which the basic resonant frequency ismeasured prior to measurement of the sample resonant frequency is notcompulsory. Rather, the sample resonant frequency can also be measuredfirst and the basic resonant frequency thereafter. In this regard it isadvantageous for the electrodes to be arranged in parallel by means of aswitch so that after the sample resonant frequency has been measured,the electrode parts can be separated from the basic resonant circuit byopening the switch; thus for measurement of the basic resonant frequencythere is no need to remove the electrode parts from the tissue.Preferably after the high-frequency application has been terminated, acontrol measurement of the basic resonant frequency is once againcarried out.

Furthermore, a sinusoidal current signal can also be used as outputsignal from the current-supply device.

The functions described for the microcontroller can also be performedentirely or in part by the control device of the high-frequency surgicalappliance. In particular it can be provided that the microcontroller ofthe measurement apparatus is integrated into the control device of thehigh-frequency surgical appliance.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

1. Method of determining the transition impedance between two electrodeparts of a subdivided neutral electrode for use in high-frequencysurgery, by providing a resonant circuit into which said two electrodeparts are incorporated in parallel with one another such that there is atransition impedance between them; measuring a resonant-frequency shiftin said resonant circuit; and determining the purely capacitivecomponent of the transition impedance using said resonant-frequencyshift.
 2. Method according to claim 1, wherein the determination of thepurely capacitive component of the transition impedance comprises thefollowing steps: providing a basic resonant circuit with a measurementinductance and a coupling capacitance; measuring a basic resonantfrequency of said basic resonant circuit; providing an expanded resonantcircuit by incorporating said transition impedance in parallel into saidbasic resonant circuit; measuring a sample resonant frequency of saidexpanded resonant circuit; and determining the capacitive component fromthe basic resonant frequency, the sample resonant frequency and themeasurement inductance, assuming that the transition impedance comprisesan ohmic resistance and a capacitive resistance that are parallel to oneanother in the basic resonant circuit.
 3. Method according to claim 2,wherein a measurement of at least one of the basic resonant frequency ofthe basic resonant circuit and the sample resonant frequency of theexpanded resonant circuit is obtained by providing a lead to therelevant resonant circuit, applying a measurement alternating voltage toone of the basic resonant circuit and the expanded resonant circuitrespectively, measuring the phase angle between current and voltage in asaid lead (26, 32) to the relevant resonant circuit, and adjusting thefrequency of the measured alternating current until current and voltageare exactly in phase with one another.
 4. Method according to claim 3,wherein a square-wave alternating-current signal of constant currentamplitude is applied to at least one of said basic resonant circuit andsaid expanded resonant circuit in order to measure respectively at leastone of the basic resonant frequency of the basic resonant circuit andthe sample resonant frequency of the expanded resonant circuit. 5.Method according to claim 4, wherein the frequency of saidalternating-current signal is adjusted by means of a timer signal inorder that the zero crossing and the frequency of saidalternating-current signal can be determined by way of the timer signal.6. Method according to claim 2, wherein a measurement alternatingvoltage is applied to at least one of said basic resonant circuit andsaid expanded resonant circuit, the voltage across at least one of saidbasic resonant circuit and said expanded resonant circuit is measuredand the frequency of the measurement alternating voltage is adjusteduntil the voltage reaches a maximal value in order to measurerespectively at least one of the basic resonant frequency of the basicresonant circuit and the sample resonant frequency of the expandedresonant circuit.
 7. Method according to claim 2, wherein the ohmiccomponent of the transition impedance is determined by providing a leadto the expanded resonant circuit, measuring the voltage across theexpanded resonant circuit and the current in said lead, and deriving theohmic component of the transition impedance from these measurements. 8.Method according to claim 2, wherein the transition impedance atpredetermined time intervals is monitored by measuring the sampleresonant frequency of the expanded resonant circuit.
 9. Method accordingto claim 1, wherein during a high-frequency surgical application thecurrent flowing away from each of the electrode parts is measured.10-16. (canceled)